The present invention generally relates to gas separators and in particular to a pressure swing adsorption type gas separator system for separating the air into nitrogen and oxygen.
A pressure swing adsorption type gas separator system is used for separation of the air into nitrogen and oxygen by using an adsorbent such as a molecular sieving carbon. The molecular sieving carbon is a porous carbon having extremely fine pores (less than 4 A in diameter) and preferentially adsorbs oxygen in the air on the surface of the pore when the air is supplied with a substantially high pressure. Further, such a molecular sieving carbon releases the oxygen adsorbed on it when the pressure is returned to a normal atmospheric pressure or the carbon is subjected to a reduced pressure. The pressure swing adsorption type gas separator system has an advantage in that the structure is relatively simple and it produces a nitrogen gas having a purity suitable for use in preservation of food and the like at a significantly reduced cost.
A typical pressure swing adsorption type gas separator system used for production of nitrogen gas generally uses an adsorption column filled with the molecular sieving carbon and produces the nitrogen gas by repeatedly performing an adsorption step in which compressed air is supplied to the adsorption column and oxygen in the compressed air is removed as a result of the adsorption of oxygen by the molecular sieving carbon, and a releasing step in which the adsorption column is depressurized by releasing the pressure in the adsorption column or by depressurizing the adsorption column by a pump and the like so that the oxygen adsorbed on the molecular sieving carbon is released. The adsorption step and the releasing step are repeated with an optimal repetition period called a cycle time. As a result, the oxygen molecule is removed from the air due to the adsorption by the molecular sieving carbon each time the pressure swing adsorption type gas separator system performs the adsorption step and the desired nitrogen gas is obtained as a residual gas which is a gas remaining in the adsorption column after the removal of oxygen.
As the nitrogen gas is obtained only when the pressure in the adsorption column is increased, the nitrogen gas is obtained only intermittently from the adsorption column. Therefore, the actual pressure swing adsorption type gas separator system uses a tank for storage of the nitrogen gas thus obtained so that the nitrogen gas having a constant pressure is obtained continuously at the site where the nitrogen gas is used.
At an initial state when the pressure swing adsorption type gas separating system begins to operate, the tank usually contains the ordinary air. After the start of operation, the nitrogen in the air is separated or extracted and supplied to the tank through a valve connecting the adsorption column and the tank. As a result, the concentration of nitrogen in the tank gradually increases and finally reaches a purity sufficient for use.
The time interval in which the pressure in the adsorption column is held at a high level in correspondence with the adsorption step, is generally determined by factors such as a flow rate of air flowing into the adsorption column, capacity and specification of air compressor to be used and the like. In a conventional pressure swing adsorption type gas separating system, the period of repetition for one cycle of operation (comprising the adsorption step and the releasing step) of the gas separating system is fixed at an optimum value such as 120 seconds by taking these factors into consideration so that the nitrogen gas having the desired purity is obtained. Such a period of repetition will be referred to hereinafter as a cycle time. However, the conventional pressure swing adsorption type gas separating system has a problem in that it requires a relatively long preliminary running time which is a running time required for the gas separating system to be operated from an initial state at which the operation of the gas separator system is started and the tanks is filled with the ordinary air until a time when the gas in the tank reaches a satisfactory level of purity.
The applicant made a series of experiments in order to eliminate or minimize this problem by variously changing the cycle time. As a result, the following was found:
(1) The running time of the system required for the purity of the nitrogen gas in the tank to reach a level which allows a relatively high concentration of oxygen such as one percent becomes shorter when the cycle time is reduced. However, it is difficult or impossible to obtain a high purity nitrogen gas which contains substantially less oxygen as long as the reduced cycle time is used.
(2) After the purity of the nitrogen gas has reached the aforementioned level, the adsorption steadily proceeds further when the cycle time is increased. By doing so, a nitrogen gas having a purity substantially exceeding 99.5% is obtained.